Thermal Analysis of Wax-Based Formulations of Personal Care Products

Each thermal profile is unique on both heating and cooling ramps, even though the samples are all comprised of 41-100% petrolatum. Therefore, DSC could help differentiate formulations using the same primary component. The results show that while the bulk ingredient may remain the same, the final products might process and store quite differently. During the second heat the baseline is more easily distinguished by starting at sub-ambient temperatures and it becomes apparent that some melting begins at temperatures of 0 °C and lower.

Many industrial standards exist as a starting point to help manufacturers consistently report similar information. For example, ASTM D4419 is a reference specific to analyzing petroleum-based products by DSC. [1] A general characterization may use a single temperature cycle to determine transition temperatures. However, additional experiments are possible to help further understand the material.

Most standard methods are adapted for broad applicability and utilize moderate heating rates to ensure enough signal. From Equation 1, note that the heat flow signal (dQ/dt) is proportional to ramp rate (dT/dt).

For some applications, slower ramp rates are desirable. A benefit of the Discovery DSC Instrument and Tzero Technology is the ability to use both small sample sizes and slow ramping rates without sacrificing data quality. One such example is wax appearance temperature determination, a DSC method analogous to the cloud point, where slower cooling ramps can be preferable for increased precision in the measurement [2, 3].

An additional method was conducted at a slower ramp rate of 2 °C/min. The sample was first cycled from 20 °C to 80 °C and subsequently cooled to -40 °C. Upon cooling, the extrapolated onset temperature was used to identify the onset of crystallization or outwaxing. For clarity, each sample is presented separately in Figure 3. When analyzing the data in TRIOS™ software, tangents to the baseline and peak were automatically fit based on the data selected. The user can adjust and select parameters manually as desired. The resulting onset (T_o) and peak maximum temperatures (T_c) are shown in Table 3.

Since the apparent crystallization peak of each sample is complex, it can be beneficial to have multiple techniques and methods of characterization for a more complete understanding of the material. An analogous study was completed using the Discovery HR Rheometer and microscope accessory and the same four materials, providing complementary data. Rheological studies have shown after heating petrolatum to either a modest 40 °C or above the melt at 80 °C and subsequently cooling back to ambient, there is increased rigidity compared to the material as received [4].

Table 3. Outwaxing temperature from DSC cooling at 2 °C/min

A multiple cooling rate method was created where the sample was heated at 10 °C/min to 90 °C, cooled at X °C/min to -90 °C, equilibrated at -90 °C, for different values of X = 10, 7, 5, and 2 sequentially. To more easily compare the impacts of the cooling rate, the data was plotted in heat capacity units in Figure 4. In general, the peak temperature increases with decreasing ramp rate, but the change is small in magnitude. The shoulder in each sample is most variable with respect to the different cooling rates. These screening results highlight the analytical capability of DSC to interrogate how different process conditions could impact the final structure of material.

It is well known that thermal history has implications for the structure of many materials. Generally, as ramp rates are selected during method development, users may choose fast cooling rates or quenching to yield a higher amorphous content, or conversely choose slow cooling to promote crystallization. Petrolatum is a broadly used excipient, so it has previously been used as a model material. While outside the direct scope of this paper, there is discussion in the literature of the morphological impacts of cooling rate on crystalline structure using complementary techniques, including rheology and x-ray scattering. The microstructure was examined by small angle x-ray diffraction and found that while there are similar amounts of structured material regardless of history, the ordering or layering is different on cooling than the structure that was lost during the initial heating. This observation correlated with a rheological study where the fast cooled (10 °C/min) petrolatum was found to have a higher yield stress than slow cooled petrolatum (0.1 °C/min) [5].

Both the source of petrolatum (i.e. different vendors) and the manufacturing process of pharmaceutical formulations have previously been correlated to significant changes in the in vitro release of qualitatively and quantitatively similar ophthalmic ointments [6]. Thermal and rheological methods can be used as an early indicator in quality control and during development. If pharmaceutical ointments have APIs that require precise dosing, investigating the physicochemical properties can provide valuable insights. Further evaluating the physical properties imparted by process conditions can be a consideration for both product usability and efficacy.